Patch "mm: memcontrol: use special workqueue for creating per-memcg caches" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

mm: memcontrol: use special workqueue for creating per-memcg caches

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mm-memcontrol-use-special-workqueue-for-creating-per-memcg-caches.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Vladimir Davydov <[email protected]>
Date: Mon, 12 Dec 2016 16:41:29 -0800
Subject: mm: memcontrol: use special workqueue for creating per-memcg caches

From: Vladimir Davydov <[email protected]>

commit 13583c3d3224508582ec03d881d0b68dd3ee8e10 upstream.

Creating a lot of cgroups at the same time might stall all worker threads with kmem cache creation works, because kmem cache creation is done with the slab_mutex held. The problem was amplified by commits 801faf0db894 ("mm/slab: lockless decision to grow cache") in case of SLAB and 81ae6d03952c ("mm/slub.c: replace kick_all_cpus_sync() with synchronize_sched() in kmem_cache_shrink()") in case of SLUB, which increased the maximal time the slab_mutex can be held.

To prevent that from happening, let's use a special ordered single threaded workqueue for kmem cache creation. This shouldn't introduce any functional changes regarding how kmem caches are created, as the work function holds the global slab_mutex during its whole runtime anyway, making it impossible to run more than one work at a time. By using a single threaded workqueue, we just avoid creating a thread per each work. Ordering is required to avoid a situation when a cgroup's work is put off indefinitely because there are other cgroups to serve, in other words to guarantee fairness.

---
 mm/memcontrol.c |   15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

@@ -2152,6 +2152,8 @@ struct memcg_kmem_cache_create_work {
 	struct work_struct work;
 };
 
+static struct workqueue_struct *memcg_kmem_cache_create_wq;
+
 static void memcg_kmem_cache_create_func(struct work_struct *w)
 {
 	struct memcg_kmem_cache_create_work *cw =

@@ -2183,7 +2185,7 @@ static void __memcg_schedule_kmem_cache_
 	cw->cachep = cachep;
 	INIT_WORK(&cw->work, memcg_kmem_cache_create_func);
 
-	schedule_work(&cw->work);
+	queue_work(memcg_kmem_cache_create_wq, &cw->work);
 }
 
 static void memcg_schedule_kmem_cache_create(struct mem_cgroup *memcg,

@@ -5786,6 +5788,17 @@ static int __init mem_cgroup_init(void)
 {
 	int cpu, node;
 
+#ifndef CONFIG_SLOB
+	/*
+	 * Kmem cache creation is mostly done with the slab_mutex held,
+	 * so use a special workqueue to avoid stalling all worker
+	 * threads in case lots of cgroups are created simultaneously.
+	 */
+	memcg_kmem_cache_create_wq =
+		alloc_ordered_workqueue("memcg_kmem_cache_create", 0);
+	BUG_ON(!memcg_kmem_cache_create_wq);
+#endif
+
 	hotcpu_notifier(memcg_cpu_hotplug_callback, 0);
 
 	for_each_possible_cpu(cpu)

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/mm-memcontrol-use-special-workqueue-for-creating-per-memcg-caches.patch